The transport and separation of Zn2+ and Cd2+ from binary sulfate solutions in a supported liquid membrane using di(2-ethylhexyl)phosphoric acid (D2EHPA) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) as mobile carriers was studied. Batch solvent extraction experiments were conducted to obtain the reaction stoichiometries. Experiments were performed at different metal concentrations (1.4-14.5 mol m(-3)), metal concentration ratios (0.4-9.2), pH (2-5), and carrier concentrations (0.1-0.6 mol dm(-3)). A mass transfer model was proposed that considers diffusion in the aqueous feed and strip stagnant layers, and within the membrane. The interfacial reactions were assumed to approach equilibrium instantaneously. It was shown that the proposed model was applicable for binary Zn2+/Cd2+ systems (standard deviation, 5%). The larger separation factors of Zn2+ over Cd2+ with PC88A than D2EHPA under equilibrium (batch solvent extraction) and non-equilibrium (liquid membrane) conditions were also evaluated and discussed. (C) 2004 Society of Chemical Industry.